The naphtha from petroleum industry contains varying amount of its constituent's viz. paraffins, naphthenes, aromatics and olefins in different proportion. Naphtha is used for different purposes depending on their boiling range and composition. They are mainly used in production of hydrogen, olefin, aromatics like Benzene, Toluene and Xylene, and as solvent in commercial and food industries. Narrow cut naphtha in the range of 63° C. to 69° C. with aromatics, mainly benzene, less than 650 ppm by weight known as food grade hexane is widely used in food industry. This low aromatic requirement of hexane is because of its application in the extraction of edible oil. Thus it is necessary to separate aromatics from the non aromatics phase to the extent possible.
The naphtha cut contains aromatics in the range of 3-15% and boiling point range from 50° C. to 140° C. depending on the petroleum crude and source of naphtha. Several processes such as catalytic hydrogenation, adsorptive dearomatisation, catalytic distillation and solvent extraction have been currently followed in industry for reduction of benzene in naphtha for the production of food grade hexane.
The catalytic hydrogenation involves the hydrogenation of benzene in liquid phase with the help of suspended solid. U.S. Pat. No. 3,505,421 describes a process of hydrogenating benzene in liquid phase by means of molecular hydrogen in the presence of suspended solid catalyst. U.S. Pat. No. 5,254,763 also describes a process for selective hydrogenation of benzene using water soluble organo-metallic hydrogenation catalyst. This hydrogenation process involves hydrogen at elevated temperature and pressure. The hydrogenation process also involves the removal of sulfur before the reaction, which makes this process more expensive.
The adsorptive dearomatisation was achieved using activated carbon, alumina, and zeolite materials. U.S. Pat. No. 3,963,934 claims the separation of aromatics from process streams by use of a molecular sieve. Other U.S. Pat. Nos. 2,728,800; 2,847,485; and 2,856,444 describes the use of silica gel for adsorbing aromatics from a process stream, followed by desorption by use of a liquid hydrocarbon. U.S. Pat. No. 5,294,334 describes the process for selectively removing benzene from gasoline boiling range process streams using aluminosilicate zeolite material.
The catalytic distillation for benzene removal is another method/technology. U.S. Pat. No. 7,501,549 claims the reduction in benzene from gasoline can be achieved by feeding a gasoline fraction with an alcohol and ether to a catalytic distillation column with one reaction zone containing an alkylation catalyst. Then the C6 hydrocarbons are separated from C7+ hydrocarbons.
The catalytic hydrogenation is another method for benzene removal where benzene reduction, olefin saturation and sulfur reduction occurs simultaneously. U.S. Pat. No. 6,153,805 discloses the catalytic hydrogenation of benzene to produce cyclo-hexane in the presence of metal catalyst. US Patent Application 2002/24395 discloses the production of food grade hexane by hydrogenation, using Ni supported alumina catalyst.
U.S. Pat. No. 4,428,829 reports the production of food grade hexane by separating aromatics and non-aromatics from heavy hydrocarbon stream by extraction process. The Indian patent applications 1224/DEL/1994 & 788/DEL/1994 discloses the separation of the aromatics and non-aromatics from naphtha and kerosene range fraction by extraction. The raffinate phase is water washed to remove the solvent carryover. The U.S. patent application 2001/82750 discloses the process for removal of aromatics from petroleum streams like naphtha, kerosene and gasoil through extraction by using solvents like NMP, Sulfolane and glycol. In all the above said process, solvent is recovered by distillation.
U.S. Pat. No. 3,551,327 discloses a process for recovery of aromatics from vapour to liquid phase i.e by extractive distillation, further it also focuses on raffinate water wash and recovery of sulfolane from water. US patent application 2001/82750 discloses a process for extraction of aromatics from petroleum fraction like heavy naphtha, kerosene and gas oil. U.S. Pat. Nos. 3,942,765, 4,314,974 discloses a process for the removal of metal ions from aqueous solution by using solvent in different type of static mixer and then whole process is confined to static mixer alone.
The processes for producing food grade hexane containing less than 100 ppm benzene is hydrogenation (U.S. patent application 2002/24395) and adsorption (U.S. Pat. No. 4,567,315). Refineries producing food grade hexane by solvent extraction process need to put up separate unit to employ these processes which is cost intensive.
Specifications for food grade hexane with respect to benzene content are becoming stringent day by day. Refineries producing food grade hexane by solvent extraction process need to switch to alternative processes like catalytic hydrogenation and adsorption which calls for additional capital investment. Thus there is a need for a cost effective process for obtaining a food grade hexane having less than 100 ppm of aromatic compound.